Fix TypeError with gcc compiler in deprecated MacOS

In some deprecated MacOS, e.g.10.13 High Sierra, when compiling formulae with --cc=gcc-XX, homebrew will raise "TypeError: Return value: Expected type Symbol, got type String with value".
Convert "other" string to symbol to solve this.

Library/Homebrew/extend/ENV/shared.rb

@@ -348,7 +348,7 @@ module SharedEnvExtension
     COMPILER_SYMBOL_MAP.fetch(value) do |other|
       case other
       when GNU_GCC_REGEXP
-        other
+        other.to_sym
       else
         raise "Invalid value for #{source}: #{other}"
       end